Low loss silicon oxynitride optical waveguide, a method of its manufacture and an optical device

ABSTRACT

The invention relates to an optical waveguide for guiding light in a predefined wavelength range, the optical waveguide comprising core and cladding regions for confining light, the core and/or cladding region or regions being formed on a substrate and comprising material of the stoichiometric composition Si a O x N y X z H. The invention further relates to a method of manufacturing an optical waveguide, an optical waveguide obtainable by the method and an optical device comprising such a waveguide. The object of the present invention is to provide an optical waveguide with low optical loss due to a reduced hydrogen bond-originated absorption. The problem is solved in that X is selected from the group of elements B, Al, P, S, As, Sb and combinations thereof, and the ratio y/z is larger than 1. This has the advantage that a low optical absorption in the waveguide may be achieved, possibly over a broad wavelength range. Further, a relatively low annealing temperature may be used yielding a relatively low induced strain whereby a low birefringence may be achieved. The optical waveguide may e.g. be manufactured by PECVD, which is ideal for the further processing of low loss waveguides. Waveguides according to the invention show superior transmission characterized with losses below 0.05 dB/cm between 900 nm and 1600 nm. In particular the absorption due to the second overtone of the Si:N—H vibration may be lowered to a value below the detection level. The invention may e.g. be used for the optical communications systems, in particular for branching components (e.g. splitters) and components for wavelength division multiplexing (WDM) systems, e.g. telecommunication systems, fibre-to-the-home, etc.

TECHNICAL FIELD

This invention relates to the manufacture of high quality optical films.

The invention relates specifically to an optical waveguide for guidinglight in a predefined wavelength range, the optical waveguide comprisingcore and cladding regions for confining light, the core and/or claddingregion or regions being formed on a substrate, and the whole or a partof the core and/or cladding region or regions comprising material of thestoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v).

The invention furthermore relates to: A method of manufacturing anoptical waveguide for guiding light in a predefined wavelength range,the optical waveguide comprising core and cladding regions for confininglight, to an optical waveguide obtainable by the method and to anoptical device comprising an optical waveguide.

This invention can be applied to all types of optical devices based onindex guiding waveguide layers as well as photonic band gap relatedwaveguide technologies. The invention may e.g. be useful in applicationssuch as optical communication systems, in particular for branchingcomponents (e.g. splitters) and components for wavelength divisionmultiplexing (WDM) systems, e.g. telecommunication systems,fibre-to-the-home-systems, etc.

BACKGROUND ART

It is well known that it is difficult to fabricate optically transparentsilica based waveguides with sufficiently low losses over a broad rangeof wavelengths. The commercially mature planar glass on siliconwaveguide technology is typically based on low-index contrast (e.g. lessthan 0.7%), the index difference between core and cladding beingcalculated as:(Δn/<n>)·100%=100%·2·(n ₁−n₂)/(n₁+n₂).

This more or less standard low-index technology platform ensures planarwaveguide components with low propagation loss (<0.05 dB/cm) and lowfiber-to-chip coupling losses (e.g. <0.3 dB/facet). The refractive indexdifference between the waveguide core and cladding is generally achievedby doping the silica core material with higher refractive index oxidessuch as germanium, phosphorous oxide, titanium oxide, etc., in order toraise the refractive index above that of the surrounding cladding.

Increasing the index above 0.7% will allow for smaller bending radiiwithout increasing the bending loss, and hence, smaller devices may befabricated (cf. e.g. “A Low-Loss, compact wide-FSR-AWG using PlanarLightwave Circuit Technology”, C. Doerr, FJ1 OFC 2003). This will allowfor more devices per wafer, or alternatively create space for morecomplex components with higher functionalities. Higher index contrastwill eventually also open up for devices which cannot be made at a lowerindex contrast such as planar devices utilising a photonic band gap(PBG) effect. Recent developments in the design and fabrication ofvisible PBG waveguide devices in Si₃N₄ type materials have beendiscussed extensively by M. D. B. Charlton, et al., J. of MaterialsScience: Materials in Electronics 10 (1999) p. 429-440 (and referencesherein).

As a promising production platform for higher index waveguides, anoxynitride (SiO_(x)N_(y)) type of material has been discussedextensively in the literature. It has been known for a long time, andshown by several groups, that a SiO_(x)N_(y) type material can befabricated with a tuneable refractive index which can be varied betweenthat of SiO₂ (1.455) and that of Si₃N₄ (2.02) by conventional depositiontechniques (such as chemical vapour deposition (CVD), plasma enhancedCVD (PECVD), atmospheric pressure CVD (APCVD) or low pressure CVD(LPCVD) processes, cf. e.g. “Reduction of hydrogen induced losses inPECVD-SiO_(x)N_(y) optical waveguides in the near infrared”, H. Alberset al., in Proceedings of OFPW3.4, LEOS '95, IEEE Lasers andElectro-Optics Society 1995 Annual Meeting, 8th Annual Meeting. Althoughthe high-end range of the refractive indices has been shown to have ahigh tendency for crack formation upon annealing to elevatedtemperatures, it has never the less been demonstrated that it is inprinciple possible to tune the refractive index over the entire range,i.e. to achieve index differences from 0% (SiO₂) to 32.5% (Si₃N₄).

However, from the literature it is also known that SiO_(x)N_(y) filmswill contain Si:N—H bonds giving rise to an absorption peak at awavelength λ=1508 nm due to an overtone of the Si:N—H vibration locatedaround 3300 cm⁻¹ (k=1/(nλ), where n is the order of the overtone, heren=2). Even though the N—H vibration is at 1508 nm the “tail” of thispeak extends into the telecom band giving rise to absorption at 1550 nmwhich is in the middle of the telecom C-band (1530-1565 nm). Increasingthe bandwidth to include the S-(1460-1530 nm) and the E- (1360-1460 nm)and the O-band (1260-1360 nm) the Si:N—H vibrations will be even moredestroying when the aim is to fabricate low loss, high index contrastdevices working over a broad wavelength range.

The intensity of the Si:N—H absorption peak can be lowered by annealingthe as deposited SiO_(x)N_(y) film to elevated temperatures (cf. e.g.“Silicon Oxynitride Layers for Optical Waveguide Applications”, R.Germann et al., J. of Electrochemical Society, 147(6), p. 2237-2241(2000) or “Passband flattened binary-tree structured add-dropmultiplexers using SiON waveguide technology”, Ph. D Thesis by ChrisRoeloffzen, Twente, 2002 (ISBN 90-365-1803-2)). Annealing temperaturesas high as 1150° C. have been reported in the literature giving lossesat the peak maxima of around 0.6 dB/cm. Unfortunately, it is notpossible to completely remove the absorption peak by simple annealing,and furthermore, the annealing approach also has another drawback ofincreasing the stress in the film layer giving rise to a significantincrease in the birefringence of the film (the degree of birefringencebeing defined by the difference between the refractive indices n_(TE)and n_(TM) of the transverse electric (TE) and transverse magnetic (TM)modes, respectively). This is clearly an unwanted side effect ofextensive annealing.

A decrease in hydrogen-bond related loss upon annealing of SiON-typematerial is also discussed in US patent applications US-2002-0182342,US-2002-0194876 and US-2002-0178760, among others.

The variety of different CVD processes discussed in the literature canbe grouped into two different categories, i.e. a type A process usingNH₃ as one of the gasses from which the film layer is formed and a typeB process where the films are nucleated from a gas composition which isnot containing NH₃. We have mapped out various different PECVDcombinations of type A and B processes. In accord with the findings inthe literature, we find that there is a fairly high loss around 1508 nmdue to the presence of Si:N—H bonds in films based on both types ofprocesses (cf. FIG. 4). This is also the case after annealing up to 16hours at 1150° C. However, the loss due to vibrating N—H bonds isslightly lower for films formed on the basis of a type B recipe ascompared to films formed by a type A recipe. Intuitively this is alsoexcepted, since a film based on a type A recipe is expected to contain ahigher density of N—H bonds since N—H bonds are directly introduced intothe film layer through fragments of the NH₃ molecule, e.g. NH_(x), x=1,2. In accord with the literature we also saw that the absorption losscould be decreased by increasing the annealing temperature and/or theannealing time.

Unfortunately the improved losses obtained by annealing are still notlow enough for low loss broad banded telecom related components.

One way to reduce the hydrogen concentration in silicon oxynitridematerial of the stoichiometric form Si_(a)O_(x)N_(y)A_(z)M_(v)H_(u) isaccording to WO-99/44937 to incorporate penta- or hexa-valent elements(A) from Group 15, 16 of the periodic system and/or mono- or di-valentmetals (M) from groups 1, 2, 11 or 12 interstitially in the glassmatrix. This is expected to reduce the hydrogen affinity of the nitrogenatoms and therefore to reduce the optical losses due to N—H-absorption.Preferred embodiments are elements of the stoichiometric formSi_(a)O_(x)N_(y)A_(z)M_(v)H_(u) wherein z≧y and/or v≧y, i.e. theconcentration of Nitrogen is less than or equal to the concentration ofA- (e.g. P, As, etc.) or M-elements (e.g. Li, Be, Cu, Zn. etc.).

In EP-1295963 A2, EP-1273677 A2, and EP-1302792 A2 the optimization ofprocess parameters such as flow rates, pressures, temperatures, gases,etc. in various steps of a PECVD process for deposition of silica filmson a wafer and subsequent heat treatment are described, the optimizationbeing performed with a view to reduce the optical absorption due toSi:N—H and Si:O—H oscillators.

No characterisation of propagation loss measurements on a finishedplanar waveguide has been performed in WO-99/44937. In EP-1295963 andEP-1273677 it is suggested to optimise the thermal treatment whichallows the optical properties to be maintained while modifying themechanical stress of the core. There is, however, no clear evidence fora correlation between optical loss and mechanical stress of the corelayer, since the stress effect of the upper cladding layer is notconsidered in the spectroscopy characterisation of the core layer.

DISCLOSURE OF INVENTION

The object of the present invention is to provide an optical waveguidewith low optical loss due to a reduced hydrogen bond-originatedabsorption. It is a further object to provide an optical waveguide withlow optical loss due to absorption in a wavelength range used foroptical transmission. In an embodiment of the invention, it is a furtherobject to lower or remove absorption peaks due to hydrogen bonds in anoptical waveguide. In an embodiment of the invention, it is a furtherobject to lower or remove absorption peaks due to N—H bonds in anoptical waveguide. In an embodiment of the invention, it is a furtherobject to lower or remove absorption peaks due to O—H bonds in anoptical waveguide. In an embodiment of the invention, it is a furtherobject to lower or remove absorption peaks due to Si—H bonds in anoptical waveguide.

The objects of the invention are achieved by the invention described inthe accompanying claims and as described in the following.

An optical waveguide for guiding light in a predefined wavelength range,the optical waveguide comprising core and cladding regions for confininglight, the core and/or cladding region or regions being formed on asubstrate, and the whole or a part of the core and/or cladding region orregions comprising material of the stoichiometric compositionSi_(a)O_(x)N_(y)X_(z)H_(v) is provided by the invention wherein X isselected from the group of elements B, Al, P, S, As, Sb and combinationsthereof, and the ratio y/z is larger than 1, such as larger than 1.2,such as larger than 1.5, such as larger than 1.8, such as larger than2.0, such as larger than 2.5, such as larger than 3.0, such as largerthan 3.5, such as larger than 4.0, such as larger than 4.5, such aslarger than 5.0, such as larger than 5.5, such as larger than 6.0, suchas larger than 7.0, such as larger than 8.0.

An advantage of the invention is that a low optical absorption in thewaveguide may be achieved. In an embodiment of the invention, a lowabsorption in the waveguide may be obtained over a broad wavelengthrange, e.g. in the range 1530-1565 nm. Further, in an embodiment of theinvention, a relatively low annealing temperature may additionally beused yielding a relatively low induced strain whereby a lowbirefringence may be achieved.

The present invention demonstrates that is possible to make an opticalwaveguide with low optical absorption properties in the S-, C-, L- andO-bands. In particular, it is possible to lower the density of Si:N—Hbonds to provide an absorption below 0.1 dB/cm (such as below 0.05dB/cm) in a Si_(a)O_(x)N_(y)X_(z)H_(v) type material where y>z, i.e. theconcentration of X (e.g. P) is less than the concentration of N.

In an embodiment of the invention, it is further possible to tune theinherent stresses by adjusting the y/z ratio or by adding a thirdelement or a combination of elements. In an embodiment the amount ofPhosphorus is used to optimize (e.g. to minimize) the inherent stressesof the optical waveguide.

In the present context, the term “waveguide” is taken to mean anyelongate guide structure which permits the propagation of a wavethroughout its length despite diffractive effects, and possiblycurvature of the guide structure. “An optical waveguide” based on totalinternal reflection is defined by an extended region of increased indexof refraction relative to the surrounding medium. “An optical waveguide”based on a photonic band gap is defined by an extended core regionsurrounded by a photonic band gap material comprising a periodic patternof holes or a periodic pattern of high index material. The strength ofthe guiding, or the confinement, of the wave depends on the wavelength,the index difference and the guide width. Stronger confinement leadsgenerally to narrower modes. An optical waveguide may support multipleoptical modes or only a single mode, depending on the strength of theconfinement. In general, an optical mode is distinguished by itselectromagnetic field geometry in two dimensions, by its polarizationstate, and by its wavelength. The polarization state of a wave guided ina birefringent material or an asymmetric waveguide is typically linearlypolarized. However, the general polarization state may contain acomponent of nonparallel polarization as well as elliptical andunpolarized components, particularly if the wave has a large bandwidth.If the index of refraction difference is small enough (e.g.Δn=n₁−n₂=0.036) and the dimension of the guide is narrow enough (e.g.width W=3 μm), the waveguide will only confine a single transverse mode(the lowest order mode) over a range of wavelengths. If the waveguide isimplemented on the surface of a substrate so that there is an asymmetryin the index of refraction above and below the waveguide, there is acutoff value in index difference or waveguide width below which no modeis confined. A waveguide may be implemented in a substrate (e.g. bydiffusion into the substrate), on a substrate (e.g. by applying acoating and etching away the surrounding regions, or by applying acoating and etching away all but a strip to define the waveguide),inside a substrate (e.g. by contacting or bonding several processedsubstrate layers together). The optical mode which propagates in thewaveguide has a transverse dimension which is related to all of theconfinement parameters, not just the waveguide width.

The width and height of a waveguide element is in the present contexttaken in a transversal cross section of the waveguide core (i.e. in across section perpendicular to the intended direction of light guidanceof said waveguide core elements at the location of a width measurement),the width being a dimension of the core region of the waveguide elementin question in a direction parallel to a reference plane defined by theopposing, substantially planar, surfaces of the substrate (x-directionin FIG. 6), the height being a dimension of the core region of thewaveguide element in question in a direction perpendicular to thereference plane (in a direction of growth, y-direction in FIG. 6).

The term “the stoichiometric composition’ of a material” reflects therelative number of units of the elements in question present in thematerial, e.g. Si_(0.97)O_(1.91)N_(0.09)P_(0.03) defining a materialwherein (on average over a given volume of the material) for each 97silicon atoms, 191 oxygen atoms, 9 nitrogen atoms and 3 phosphorus atomsare present. The suffixes or numbers a, x, y, z, v in the stoichiometriccomposition Si_(a)O_(x)N_(y)X_(z)H_(v) represent the molarconcentrations of the constituent elements calculated relative to thesum a+x+y+z+v, e.g. the relative concentration c(N) of the elementnitrogen in the composition Si_(a)O_(x)N_(y)X_(z)H_(v) equalsy/(a+x+y+z+v). In the present context, the atomic concentration of anelement Q (e.g. H) measured in atomic % (at. %) is taken to meanc(Q)·100 (i.e. for hydrogen c(H)·100=v·100/(a+x+y+z+v) in aSi_(a)O_(x)N_(y)X_(z)H_(v) material).

The volume—termed a ‘given volume’ above—over which the composition isaveraged—is preferably the total volume of the sample or layer having agiven intended stoichiometric composition, e.g. the volume of the core.Alternatively, the ‘given volume’ may be a representative part of thetotal volume, i.e. a macroscopic part of the sample, such as more than1% of the total volume of the part or layer in question, i.e.statistically large enough to allow a meaningful average value.Alternatively, the ‘given volume’ may be a volume that is at least 5times the volume expected to comprise one unit of the stoichiometriccomposition in question, such as at least 100 times, such as at least1000 times. Alternatively, the ‘given volume’ may be defined by adimension comparable to the wavelength λ of the propagating wave inquestion, such as 2 times λ, such as 10 times λ.

The atomic density (atoms/unit volume, e.g. atoms/cm³) of the differentelements in a given sample may e.g. be determined by Secondary Ion MassSpectrometry (SIMS) measurement or by an energy-dispersive X-rayanalysis (EDX) measurement. The basic principles of both techniques arediscussed extensively in various textbooks, see e.g. “Fundamentals ofsurface and thin films analysis”, L. C. Feldman, J. W. Mayer, ISBN0-441-00989-2, wherein—for example—quantitative analysis down to anaccuracy of about 1% by EDX is discussed. EDX is characterized by beinga surface sensitive tool with electron penetration depths between 5 and100 Å, depending on the energy of the incoming electron.

A connection between atomic concentration and relative molarconcentration may be estimated by assuming or measuring a certain massdensity ρ(SiONXH) of the resulting material (H may optionally beneglected due to its small contribution to the mass density). The atomicdensity N_(at) for a given element Q (Q=Si, O, N, X, H) is given by theformulaN _(at)(Q)=c(Q)·ρ(SiONXH)·N _(a) /M _(tot)where c(Q), as mentioned above, is the relative concentration of theelement Q in the composition Si_(a)O_(x)N_(y)X_(z)H_(v), N_(a) isAvogadro's number (the number of atoms or molecules in a mole) andM_(tot)=a·M_(Si)+x·M_(o)+y·M_(N)+z·M_(X)+v·M_(H) is the mole mass (unitmass/mole, e.g. g/mole) of the Si_(a)O_(x)N_(y)X_(z)H_(v)-material,where M_(Q) is the atomic mass of element Q (e.g. M_(Si)=28.086 g/mole).

In an embodiment, the material further comprises Ge. In other words, Gemay be present in combination with one or more of the elements X=B, Al,P, S, As, Sb. This has the additional advantage of providing thepossibility to fine tune the refractive index and to tailor thephotosensitivity of the material.

In an embodiment of the invention, the optical waveguide comprises acladding region surrounding the core region. In an embodiment of theinvention, the cladding and the core region comprises material of thestoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v) and wherein X isselected from the group of elements B, Al, P, S, As, Sb and combinationsthereof (and/or in combination with Ge), and the ratio y/z is largerthan 1. The index difference between core and cladding may e.g. beprovided by changing the y/x ratio (i.e. changing the[N]/[O]-concentrations, e.g. by changing the y/x ratio of the feed gasin a CVD-process). In an embodiment of the invention, the core and/orcladding region or regions are constituted essentially of material ofthe stoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v). The term‘constituted essentially of material . . . ’ is taken to refer to theelements of significance to the merits of the invention.

In an embodiment of the invention the, core region comprises material ofthe stoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v) and wherein Xis selected from the group of elements B, Al, P, S, As, Sb andcombinations thereof (and/or in combination with Ge). In thisembodiment, the cladding region or regions may comprise any type ofmaterial having an appropriate refractive index, e.g. a PBSG type glassor a PBG region.

In an embodiment of the invention the, waveguide comprises a core regionand two or more surrounding cladding regions (e.g. two ‘concentric’cladding regions wherein at least one of the cladding regions comprisesmaterial of the stoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v)and wherein X is selected from the group of elements B, Al, P, S, As, Sband combinations thereof (and/or in combination with Ge). In thisembodiment, the core region may comprise any type of material having anappropriate refractive index, e.g. a PBSG type glass or a PBG type glassor a Ge doped silica glass. In this embodiment, a further claddingregion or regions may comprise any type of material having anappropriate refractive index, e.g. a PBSG type glass or a PBG region orSiO₂.

In an embodiment of the invention, the ratio y/z is in the range 1.0 to100, such as 1.0 to 20, such as 1.0 to 10, such as 1.5 to 8.0, such as2.0 to 4.0, such as 2.5 to 3.5.

In an embodiment of the invention, the ratio y/z is essentially equal to3.

In an embodiment of the invention, the ratio y/z is essentially equal to7.

The term ‘essentially equal to’ is in the present context taken to meanbeing within 10% of the value in question, such as within 5%, such aswithin 1%.

In an embodiment of the invention, the number ‘a’ defining the relativeconcentration of the element Si is in the range 0.1 to 3.5, such as inthe range 0.5 to 3.5 such as in the range 0.9 to 1.1 (e.g. SiO₂-like) orin the range 2.9 to 3.1 (e.g. Si₃N₄-like).

In an embodiment of the invention, the number ‘x’ defining the relativeconcentration of the element O is in the range 0 to 2.5, such as in therange 1.9 to 2.1 (e.g. SiO₂-like) or in the range 0 to 0.1 (e.g.Si₃N₄-like).

In an embodiment of the invention, the number ‘y’ defining the relativeconcentration of the element N is in the range 0 to 4.5, such as in therange 3.9 to 4.1 (e.g. Si₃N₄-like) or in the range 0 to 0.5, such as inthe range 0.02 to 0.3, such as in the range 0.03 to 0.2, such as in therange 0.04 to 0.10.

In an embodiment of the invention, the number ‘z’ defining the relativeconcentration of the element X selected from the group comprising B, Al,P, S, As, Sb and combinations thereof is in the range 0 to 0.3, such asin the range 0 to 0.2, such as in the range 0.005 to 0.2, such as in therange 0.01 to 0.10.

In an embodiment of the invention, the number ‘v’ defining the relativeconcentration of the element H is defined by 0≦v<0.05.

In an embodiment of the invention, ‘a’ is in the range 0.8 to 1.2 and‘x’ is in the range 1.8 to 2.2 and ‘y’ is in the range 0.01 to 0.5 and‘z’ is in the range 0.005 to 0.2. This illustrates the situation of aSiO₂-based SiONX-composition with comparatively small relative amountsof N and X-elements.

In an embodiment of the invention, ‘a’ is in the range 2.8 to 3.2 and‘x’ is in the range 0.01 to 0.5 and ‘y’ is in the range 3.8 to 4.2 and‘z’ is in the range 0.005 to 0.2. This illustrates the situation of aSi₃N₄-based SiONX-composition with minor relative portions of O andX-elements.

In an embodiment of the invention, the number ‘a’ defining the relativeconcentration of the element Si is in the range 0.9 to 1.1, the number‘x’ defining the relative concentration of the element O is in the range1.9 to 2.1, the number ‘y’ defining the relative concentration of theelement N is in the range 0.015 to 0.12, and the number ‘z’ defining therelative concentration of the element X is in the range 0.005 to 0.04.

In an embodiment of the invention, the waveguide core and/or claddinglayers comprise material of the stoichiometric compositionSi_((1−z))O_((2−y))N_(y)X_(z) wherein X is an element from the groupcomprising B, Al, P, S, As, Sb or a combination thereof.

In an embodiment of the invention, the waveguide core and/or claddinglayers consists essentially of material of the stoichiometriccomposition Si_((1−z))O_((2−y))N_(y)X_(z) wherein X is an element fromthe group comprising B, Al, P, S, As, Sb or a combination thereof.

In an embodiment of the invention, the waveguide core and/or claddinglayers comprise material of the stoichiometric compositionSi_((1−z))O_((2−y))N_(y)P_(z). In an embodiment of the invention,0≦y<0.2 and 0<z≦0.1. In embodiments of the invention, the atomic densityof silicon N_(at)(Si) in the range 4.5·10²¹<N_(at)(Si)<1.3·10²², such asin the range 5.1.·10²¹<N_(at)(Si)<9.1·10²¹, the atomic density of oxygenN_(at)(O) is in the range 9.0·10²¹N_(at)(O)<2.7·10²², such as in therange 1.0·10²²<N_(at)(O)<1.8·10²², the atomic density of nitrogenN_(at)(N) is in the range 0<N_(at)(N)<2.7·10²¹, such as in the range0<N_(at)(N)<1.8·10²¹, and the atomic density of phosphorus N_(at)(P) isin the range 0<N_(at)(P)<1.3·10²¹, such as in the range0<N_(at)(P)<9.0·10²⁰.

In an embodiment of the invention, the atomic density of at least one ofthe elements Si, O, N, P in Si_((1−z))O_((2−y))N_(y)P_(z) is determinedby SIMS.

In an embodiment of the invention, the atomic density of at least one ofthe elements Si, O, N, P in Si_((1−z))O_((2−y))N_(y)P_(z) is determinedby EDX.

In an embodiment of the invention, X comprises more than one element,such as two or more, such as m in total, each having a relativeconcentration compared to the total concentration z of X termed z₁ forX(1) and z₂ for X(2), etc., and z_(m) for X(m). The term “the element orelements X of the material Si_(a)O_(x)N_(y)X_(z)H_(v) comprises at leastq₁% of the element X(1)” is in the present context taken to mean thatq₁/100=z₁/z, where z=SUM(z_(i)), i=1, 2, . . . , m, where SUM(z_(i))denotes the algebraic sum of the elements z_(i). The relativeconcentration of the element X(i) in the materialSi_(a)O_(x)N_(y)X_(z)H_(v), on the other hand, is z_(i)/(a+x+y+z+v).

In an embodiment of the invention, the element or elements X or thematerial Si_(a)O_(x)N_(y)X_(z)H_(v) comprises at least 50% phosphorussuch as at least 75% phosphorus such as at least 90% phosphorus, such as100% phosphorus.

In an embodiment of the invention, the element or elements X or thematerial Si_(a)O_(x)N_(y)X_(z)H_(v) comprises at least two elementsX(1), X(2), . . . , X(n) where n≦7, selected from the group comprisingB, Al, P, S, Ge, As, Sb of relative concentrations z₁, z₂, . . . ,z_(n), respectively, where z=z₁+z₂+z₃+ . . . +z_(n) and wherein z₁/z islarger than 0.50 such as larger than 0.75 such as larger than 0.90.

In general, the addition of P, Ge and N to silica glass base increasesthe refractive index, whereas the addition of B decreases the refractiveindex of the resulting waveguide material. In general, the addition of Band P increases the flow properties of a resulting silica glass.Interestingly, we have in connection with the present invention observedthat the addition of P to SiON lowers the refractive index, suggestingthat P might substitute N.

Recently it has been suggested (US2003/0021578) that the addition of Geto a BPSG material allows for a decrease in P while retaining the samerefractive index. As a consequence of the lower P-concentration, asmaller density of BPO₄ is formed during subsequent annealing, allowingfor an improved reflow mechanism enabling the fabrication of high-aspectratio structures without keyhole formation (i.e. without voids orinclusions in the cladding material). Similarly, an improved reflowmechanism enabling the fabrication of high-aspect ratio structureswithout keyhole formation might be expected forSi_(a)O_(x)N_(y)X_(z)H_(v) with X(1)=P, X(2)=Ge and X(3)=B.

In an embodiment of the invention, n=2 and X(1) is P and X(2) is B. Inan embodiment of the invention, n=2 and X(1) is P and X(2) is Ge. Thishas the advantage that the relative concentrations of P and B or Ge maybe used to fine tune the resulting refractive index.

In an embodiment of the invention, n=3 and X(1) is P, X(2) is B and X(3)is Ge. This has the advantage that the relative concentrations of P, Band Ge may be used to fine tune the resulting refractive index.

In an embodiment of the invention, the optical absorption peak at λ=1508nm due to Si:N—H bonds is smaller than 0.1 dB/cm, such as smaller than0.05 dB/cm, such as smaller than 0.01 dB/cm, as measured e.g. by aplanar version of a conventional cut-back-method.

The concentration of H is preferably so small that the opticalabsorption peak due to Si:N—H bonds is smaller than 0.1 dB/cm at λ=1508nm, such as smaller than 0.05 dB/cm, such as smaller than 0.01 dB/cm. Inan embodiment of the invention, the concentration of H is smaller than10000 ppm (i.e. v/(a+x+y+z+v)<10⁻²), such as smaller than 1000 ppm, suchas smaller than 100 ppm, such as smaller than 10 ppm. In an embodimentof the invention, the relative concentration of H is much smaller thanthe relative concentration of N, i.e. e.g. v<10⁻²y, such as v<10⁻³y,such as v<10⁻⁴y. In an embodiment of the invention, the relativeconcentration of H is much smaller than the relative concentration of X,i.e. e.g. v<10⁻²z, such as v<10⁻³z, such as v<10⁻⁴z, where X is anelement selected from the group comprising B, Al, P, S, As, Sb andcombinations thereof. In an embodiment of the invention, the atomicconcentration of hydrogen is less than 25 at. %, such as less than 15at. %, such as less than 5 at. %.

In an embodiment of the invention, the concentration of H is larger thanthe concentration of N or X or the concentration of N plus X (i.e. v>y,or v>z or v>y+z). However, in this case a part of the hydrogen atoms maynot contribute to the optical absorption around 1508 nm and/or may notbe bound in the material in an N—H type bond. In an embodiment of theinvention, the concentration of hydrogen atoms in the materialSi_(a)O_(x)N_(y)X_(z)H_(v) is in the range 5 to 25 at. %, such as in therange 10 at. % to 20 at. %.

The hydrogen concentration of a sample may e.g. be determined byhydrogen nuclear reaction analysis (cf. e.g. “Fundamentals of surfaceand thin films analysis”, L. C. Feldman, J. W. Mayer, ISBN0-444-00989-2).

It should be noted that a small percentage of nitrogen impurity willmostly be present in CVD deposited glass when nitrogen is used ascarrier gas or for example as part of the oxygen containing reactiongasses (i.e. N₂O or NH₃). The percentages of the nitrogen impurity insuch deposited glasses may be low and are often not measured orreported, partly because of the difficulty of ascertaining the nitrogencontent with a reasonable accuracy. The analytical detection problemrepresents one reason why the unusual properties and advantages ofchemically bound nitrogen in glasses are neither fully understood norappreciated in the industry.

In an embodiment of the invention, the core and/or cladding regioncomprises material having a refractive index in the range 1.45-2.02,such as in the range 1.45 to 1.60, such as in the range 1.48 to 1.56 ata wavelength of 1550 nm. This may e.g. be achieved by addition of minoramounts of dopant ions such as Ge or Al. This has the advantage ofallowing the manufacture of a relatively high index-waveguide whileavoiding the N—H-absorption peak.

The term “the refractive index” of a region or volume represented by aparticular cross sectional area of the waveguide is in the presentcontext taken to mean the geometrical refractive index. If the region inquestion is constituted by one homogeneous material with a specificrefractive index, the geometric refractive index is the normalrefractive index for a homogeneous material. If the region in questionis constituted by several smaller areas each of a homogeneous material,the geometric refractive index is the geometrically weighted average ofthe normal refractive indices of these smaller areas, i.e. the sum ofthe products of refractive index no and ratio A_(i)/A of the partialarea A_(i) in question to the area A of the whole region beingconsidered (i.e. SUM(n_(i)·(A_(i)/A)), i=1, 2, . . . , m, where m is thenumber of smaller (or partial) areas constituting the region beingconsidered).

In some cases the effective refractive index n_(eff) is convenientlyused to characterize properties of an optical waveguide. Instead ofconsidering the true waveguide structure with core and cladding thelight propagation may be described as a plane wave propagating in ahomogeneous medium having a refractive index n_(eff), the so-calledeffective refractive index. This effective index is rooted in eigenvalueequations originating from Maxwell's equations.

The effective refractive index of a bound mode is greater than thecladding refractive index, and lower than the core refractive index. Theeffective index is furthermore a function of the waveguide corecross-sectional geometry, see e.g. H. Nishihara et al., “OpticalIntegrated Circuits”, McGraw-Hill (1989).

An optical waveguide according to the invention may be used for guidinglight of any wavelength. In an embodiment of the invention, the opticalwaveguide is adapted to guide light in a wavelength range located in therange of 250 nm to 3.6 μm, such as in the range of 850 nm to 1800 nm. Inan embodiment of the invention, the optical waveguide is adapted toguide light comprising wavelengths in the range of 1260 nm to 1660 nm,such as in the range 1530-1565 nm, or in the range 1460-1530 nm, or inthe range 1360-1460 nm, or in the range 1260-1360 nm.

In an embodiment of the invention, the waveguide core and/or claddingfurther comprises one or more of the rare earth (RE) elements (i.e. theelements Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). In anembodiment of the invention, one or more of the RE elements are presentin concentrations in the range 50 to 5000 ppm (mole/mole) i.e. in amaterial of the stoichiometric compositionSi_(a)O_(x)N_(y)X_(z)H_(v)(RE)_(q), 50·10⁻⁶<q/(a+x+y+z+v+q)<5000·¹⁰⁻⁶.This has the advantage of enabling the formation of an active waveguide(for an amplifier or laser type functionality) in combination with thelow absorption loss features.

In an embodiment of the invention, the core and/or cladding region orregions additionally comprise Ge in sufficient amounts to minimiseinternal stress due to thermal expansion or contraction of thewaveguide. This has the advantage of enabling a reduce birefringence ofthe core and/or cladding layers In an embodiment of the invention, thematerial(s) of the core and/or cladding region or regions comprise lessthan 5 at. % Ge.

In an embodiment of the invention, the thermal expansion of one or moreof the layers constituting the core and cladding regions of thewaveguide is/are adapted to the thermal expansion of the substrate byadding one or more TE-dopant elements to said one or more layers of thewaveguide. ‘TE-dopant’ is used as an abbreviation for ‘Thermal Expansioninfluencing dopants’.

In an embodiment of the invention, the TE-dopant element or elementsis/are selected from the group of elements comprising Al, B, F, Ge, P,Ti, or combinations thereof.

In an embodiment of the invention, the TE-dopant element or elements arepresent in the core/and or cladding region or regions in molarconcentrations in the range 0 to 5%, i.e. taken relative to the sum ofthe molar concentrations of Si, O, N, X, H, (and possibly RE-dopants)and TE-dopants.

In an embodiment of the invention, the TE-dopant element or elements arepresent in the core/and or cladding region or regions in amountssufficient to provide a coefficient of thermal expansion between 1·10⁻⁷° C.⁻¹ and 15·10⁻⁷ ° C.⁻¹.

In an embodiment, the waveguide comprises Phosphorus in the core and/orin the cladding region(s).

In an embodiment, the optical waveguide comprises a buffer materialconstituting a barrier between the core and cladding regions and fullyor partially surrounding the core region. In an embodiment, a barrierlayer is only applied on top of the core region, thereby partiallysurrounding the core region. In an embodiment, a barrier layer isapplied on top as well as below the core region, thereby fullysurrounding the core region. An advantage of inserting the buffer layeror layers is that the tendency to formation of crystallites may belowered or eliminated, whereby the out-diffusion of Phosphorus from thecore region may be lowered or eliminated.

In an embodiment, the buffer material is selected from the group SiO₂,Si_(x)N_(y), (i.e. Si_(x)N_(y) may e.g. be Si₃N₄), PECVD BPSG withalternative B/P doping levels and combinations thereof.

In an embodiment of the invention, the optical waveguide takes the formof a planar waveguide formed on a substrate. In an embodiment of theinvention, the substrate is silicon. This has the advantage offacilitating large scale production. In an embodiment of the invention,the substrate is quartz. This has the advantage of inherently a lowercladding layer, and furthermore, induced stress from the substrate (e.g.silicon) is avoided.

In an embodiment of the invention, the waveguide is part of a photoniccrystal structure, allowing the propagation of electromagnetic energy ina certain wavelength range to be controlled by the introduction ofperiodic structural features providing a photonic band gap (PBG).

In an embodiment of the invention, the optical waveguide is manufactureby chemical vapour deposition (CVD), such as plasma enhanced CVD (PECVD)or Atmospheric Pressure CVD (APCVD). This has the advantage of providingan industrially proven manufacturing technology which is reliable andreadily scalable for large volumes.

Alternatively the waveguides may be manufactured by other techniquessuch as flame hydrolysis deposition or techniques for spinning materialson glass.

A method of manufacturing an optical waveguide for guiding light in apredefined wavelength range, the optical waveguide comprising core andcladding regions for confining light is furthermore provided by thepresent invention, the method comprising the steps of

A) providing a substrate,

B) forming a lower cladding layer on the substrate,

C) forming a core region of said optical waveguide on the lower claddinglayer,

D) forming an upper cladding layer to cover the core region and thelower cladding layer;

wherein the whole or a part of said waveguide core and/or claddingregion or regions comprise material of the stoichiometric compositionSi_(a)O_(x)N_(y)X_(z)H_(v) and X is selected from the group of elementsB, Al, P, S, As, Sb and combinations thereof, and wherein y>z, such aslarger than 1.2, such as larger than 1.5, such as larger than 1.8, suchas larger than 2.0, such as larger than 2.5, such as larger than 3.0,such as larger than 3.5, such as larger than 4.0, such as larger than4.5, such as larger than 5.0, such as larger than 5.5, such as largerthan 6.0, such as larger than 7.0, such as larger than 8.0.

The method is easily integrated into proven state of the artmanufacturing technologies of optical waveguides and integrated opticalcomponents having the advantages of corresponding (above mentioned)optical waveguides is provided.

In an embodiment of the invention, 0.5<a<3.5, 0<x<2.5, 0<y<4.5, 0<z<0.2.

In an embodiment of the invention, 0.5<a<3.5, 0<x<2.5, 0<y<4.5, 0<z<0.2and 0≦v<0.05.

In an embodiment of the invention, v≧0.05. In an embodiment of theinvention, v is larger than y or z or y+z. In an embodiment of theinvention, the atomic concentration of hydrogen in the materialSi_(a)O_(x)N_(y)X_(z)H_(v) is less than 25 at. %, such as less than 15at. %, such as less than 5 at. % or in the range 5 to 25 at. %, such asin the range 10 at. % to 20 at. %.

In an embodiment of the invention, 0.5<a<3.5, 0<x<2.5, 0<y<4.5, 0<z<0.2.

In an embodiment of the invention, 0.8<a<1.2, 1.8<x<2.2, 0.01<y<0.5,0.005<z<0.2.

In an embodiment of the invention, 0.9<a<1.1, 1.9<x<2.1, 0.015<y<0.12,0.005<z<0.04.

In an embodiment of the invention, 2.8<a<3.2, 0.01<x<0.5, 3.8<y<4.2,0.005<z<0.2.

In an embodiment, the material further comprises Ge. In other words, Gemay be present in combination with one or more of the elements X=B, Al,P, S, As, Sb. This has the additional advantage of providing thepossibility to fine tune the refractive index and to tailor thephotosensitivity of the material.

In an embodiment of the invention, step C) comprises the sub-steps

C1) forming a core layer on the lower cladding layer,

C2) providing a core mask comprising a core region pattern correspondingto the layout of the core region of said optical waveguide, and

C3) forming core regions using the core mask, a photolithographic and anetching process. This has the advantage of availing the use of industrystandard manufacturing processes, well-known from the manufacture ofintegrated semiconductor as well as optical circuits.

In an embodiment, a sub-step

C4) of forming a barrier layer on top of said core region pattern, andoptionally on top of the lower cladding layer not covered by the coreregion pattern;

is inserted before step D). An advantage of inserting the buffer layeror layers is that the tendency to formation of crystallites may belowered or eliminated, whereby the out-diffusion of Phosphorus from thecore region may be lowered or eliminated.

In an embodiment, a sub-step C0) of forming a barrier layer on top ofsaid lower cladding layer is inserted before step C1). This has theadvantage of facilitating the formation of a barrier layer surroundingcore region of the waveguide.

In an embodiment, a sub-step of annealing is inserted after said barrierforming step or steps C0), C4). This has the advantage of allowingreflow of the barrier layer before applying other layers, e.g. an uppercladding layer.

In an embodiment of the invention, the substrate is a silicon or quartzsubstrate.

In an embodiment of the invention, the optical waveguide is manufactureby chemical vapour deposition (CVD), such as plasma enhanced CVD (PECVD)or Atmospheric Pressure CVD (APCVD). This has the advantage of providingan industrially proven manufacturing technology which is reliable andreadily scalable for large volumes.

In an embodiment of the invention, a standard cluster tool CVD processchamber type PECVD-apparatus from Surface Technology Systems is used forthe formation of layers on the substrate.

In an embodiment of the invention,

the N₂ flow rate: is in the range 0-2000 sccm,

the N₂O flow rate is in the range 100-400 sccm,

the NH₃ flow rate is in the range 0-300 sccm,

the SiH₄ flow rate is in the range 0-30 sccm,

the 5% PH₃ in N₂ flow rate is in the range 0-50 sccm,

the power is in the range between 0 and 1200 W,

the pressure is in the range 50-500 mTorr,

the temperature is in the range 200-400° C.,

the frequency is around 380 kHz or around 13.56 MHz,

The unit sccm is short for Standard Cubic Centimeters per Minute beingdefined as a cubic centimetre at standard pressure and temperature (i.e.atmospheric pressure and 0° C.).

In an embodiment, the X=P and in i) the PH₃ flow is provided by PH₃diluted in N₂ or another carrier gas.

In an embodiment, in i) the PH₃ flow is provided by 5% PH₃ in N₂ with aflow rate of 0 to 50 sccm such as 2 to 20 sccm.

In an embodiment, X comprises P and in i) the PH₃ flow is provided byPH₃ diluted in N₂ or another carrier gas and the PH₃ flow value is usedas a stress optimization parameter for the core region.

In an embodiment, processing parameters of the PECVD process essentiallyhave the following values:

a) SiH₄ flow rate 20 sccm;

b) the N₂O flow rate 100-400 sccm;

c) the N₂ flow rate 2000 sccm;

d) the NH₃ flow rate is 100 sccm;

e) the power is 700 W;

f) the pressure is 250 mTorr;

g) the temperature 350° C.;

h) the frequency is 380 kHz;

i) 5% PH₃ in N₂ flow rate 10 sccm.

In an embodiment, in step i) the flow gas is selected among the group ofgases SiH₄, SiF₄, SiCl₄, SiF₄, Si₂H₆, SiH₂Cl₂, SiCl₂F₂, SiH₂F₂, N₂O, NO,N₂, NO₂, O₂, H₂O, H₂O₂, CO, CO₂, N₂O, NO, N₂, NO₂, NH₃, N₂, B₂H₆, AlH₃,PH₃, H₂S, SO, SO₂, GeH₄, AsH₃, or combinations thereof.

An optical waveguide obtainable by a method of manufacturing asdescribed above is moreover provided by the present invention. Such awaveguide has the same advantages as for the waveguides outlined above.

An optical device comprising an optical waveguide as defined above isfurther provided by the present invention. Examples of devices whereinwaveguides according to the invention could be useful are e.g.splitters, couplers, e.g. an arrayed waveguide grating (AWG), ageneralized Mach-Zehnder interferometer, and any other functional unitsbeing part of an optical communication system. In an embodiment,waveguides according to the invention are included in a duplexer ortriplexer. An optical triplexer is an optical component allowing thecombination of digital communication and analogue video reception via anoptical waveguide. Such a component is e.g. useful in fibre-to-the-homesolutions.

In an embodiment, an optical triplexer (or more precisely expressed‘triplex transceiver component’) gives full-duplex digital communicationover a single fiber (1310 nm upstream laser emission and 1490 nmdownstream detection) with an additional analogue video receiver (at1550 nm). The triplex transceiver may be established by hybrid mountingof active devices (laser and photo diodes) on a passive planar lightchip established by use of the glass material of the present invention.In a preferred version the triplex transceiver is hybrid mounted by useof a single step solder process for simple interface and support of highvolume assembly.

The use of the inventive glass material is advantageous in combinationwith the triplex transceiver due to the absence of the 1508 nmNH-absorption peak while maintaining the ability to use high indexcontrast waveguides giving raise to reduced chip size.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other stated features, integers,steps, components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with apreferred embodiment and with reference to the drawings in which:

FIG. 1 shows the refractive index at λ=1550 nm for the core region ofvarious optical waveguides according to the invention, before and afterannealing, respectively,

FIG. 2 shows corresponding values of layer thickness (closed symbols)and refractive index (open symbols) at λ=1550 nm for the core region ofa waveguide according to the invention as a function of annealingtemperature,

FIG. 3 shows a measure for the birefringence at λ=1550 nm of a waveguideaccording to the invention as a function of annealing temperature,

FIG. 4 shows optical absorption loss in dB/cm from λ=1500 nm to λ=1600nm for a waveguide manufactured according to different processingconditions, respectively, ‘with NH₃’, ‘without NH₃’ and a ‘new process’according to the invention,

FIG. 5 shows optical propagation loss in dB at λ=1550 nm for a waveguideaccording to the invention as a function of waveguide length (in cm) andmode (TE or TM),

FIG. 6 shows a cross section of a part of an optical component accordingto the invention,

FIG. 7 a shows an isolated waveguide without neighboring particleformation, and FIG. 7 b shows neighboring waveguides with particleformation, and

FIG. 8 shows birefringence vs. refractive index at λ=1550 nm for opticalwaveguides according to the invention manufactured using different PH₃flow rates.

The figures are schematic and simplified for clarity, and they just showdetails which are essential to the understanding of the invention, whileother details are left out. Throughout, the same reference numerals areused for identical or corresponding parts.

MODE(S) FOR CARRYING OUT THE INVENTION

A process according to the invention may be optimized as regardsproviding a waveguide with a low optical absorption around λ=1508 nmusing one or more of the hints listed below:

1. Avoid NH₃ since this is an additional hydrogen source and contains anNH-bond.

2. Lower the SiH₄ flow since this is also a potential hydrogen source.

3. Increase the plasma power since more Si—H bonds will be brokenallowing for less incorporation of hydrogen containing Si fragments ascompared to when the plasma process is driven at a lower power where itis expected that a smaller amount of Si—H bonds are broken.

4. Increase the total flow since this will flush out the reactionproducts the most undesirable being hydrogen and hydrogen containingfragments.

5. Increase the temperature since this is expected to decrease theconcentration of hydrogen in the plasma grown PECVD glass.

6. Changing the pressure will change the fragmentation degree of themolecules in the plasma and hereby changing the properties of the grownfilm.

7. Changing the frequency of the energy source driving the plasma willalter the properties of the PECVD grown film.

8. Add a small amount of (optionally diluted) PH₃.

A Combination of one or more of the above mentioned elements may resultin a total removal of the N—H peak an overtone of which causes extensiveabsorption at 1508 nm when a PECVD grown core glass is used as anoptical waveguide.

In various embodiments of the present invention, the waveguide materialscomprise a number of the following elements Si, O, N, X (‘X’=B, Al, P,S, As, Sb), H, RE-dopants (‘RE’=Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb, Lu ), TE-dopants (‘TE’=Al, F, Ge, Ti, B, P) in thestoichiometric composition Si_(a)O_(x)N_(y)X_(z)H_(v)(RE)_(q)(TE)_(p),where X denotes one or more of the elements ‘X’ for controllingrefractive index and/or optical absorption properties and/or thermalexpansion properties, RE one or more of the rare-earth elements ‘RE’ forcontrolling optical gain, and TE one or more of the elements ‘TE’ forcontrolling thermal expansion.

PECVD also known as Plasma CVD (PCVD) and Low Pressure Chemical VapourDeposition (LPCVD) are described in further detail in Hiroshi Nishihara,Masamitsu Haruna and Toshiaki Suhara “Optical integrated circuits”,McGraw-Hill Book Company, (1989), and in Chapter 3 in Marc Madou:“Fundamentals of Microfabrication”, ISBN 0-8493-9451-1, which areincorporated herein by reference.

EXAMPLE 1

A PECVD core glass has been grown on a standard PECVD apparatus (in thiscase a standard cluster tool CVD process chamber type PECVD-apparatusfrom STS (Surface Technology Systems plc of Newport, South Wales, UK) isused for the formation of layers on a silicon substrate using thefollowing parameters:

a) SiH₄ flow rate: 20 sccm

b) N₂O flow rate: 100-400 sccm

c) N₂ flow rate: 2000 sccm

d) 5% PH₃ in N2 flow rate: 10 sccm

e) Power: 700 W

f) Pressure: 250 mTorr

g) Temperature: 350° C.

h) Frequency: 380 kHz

FIG. 1 shows the refractive index at λ=1550 nm for the core region ofvarious optical waveguides according to the invention, before and afterannealing, respectively. Annealing was performed at 1100° C. for 4 hoursin a nitrogen atmosphere.

The refractive index may easily be tuned in a fairly large range andsignificantly larger than indicated in FIG. 1. The refractive indexchange is completely governed by the ratio of nitrogen to oxygen atomsin the PECVD feed gas (at constant SiH₄ flow) as indicated by the linearrelation of the refractive index as a function of the [N]/[O] ratio (cf.FIG. 1). Here [N]=2·[N₂O]+2·[N₂]+0.95·5% PH₃/N₂] and [O]=[N₂O] where[xx] denotes the flow rate of species xx in sccm.

FIG. 2 shows corresponding values of layer thickness and refractiveindex at λ=1550 nm for the core region of a waveguide according to theinvention as a function of annealing temperature. Open symbols indicaterefractive index at λ=1550 nm. Closed symbols indicate layer thicknessin μm. Atomic densities of nitrogen and phosphorus are [N]=1.30·10²¹atoms/cm³ and [P]=4.58·10²⁰ atoms/cm³, respectively, as determined bycalibrated SIMS.

The index of the as deposited PECVD glass can be modified upon annealingas seen in FIG. 2. The index change is initiated above approximately800° C. and continues beyond 1100° C.

FIG. 3 shows a measure for the birefringence at λ=1550 nm of a waveguideaccording to the invention as a function of annealing temperature.

The index change is correlated with an increase in the birefringence asseen from FIG. 3 where the difference between refractive index for TEand TM modes measured at λ=1550 nm is plotted as a function of thetemperature. Thus, it is suggested to use a relatively low annealingtemperature in order to minimize the build up of birefringence. However,the applied annealing temperature should of cause be compatible withsubsequent cladding procedure.

An optical component including a waveguide according to the presentinvention may be manufactured by a method comprising the steps of

A) providing a substrate,

B) forming a lower cladding layer on the substrate,

C1) forming a core layer on the lower cladding layer,

C2) providing a core mask comprising a core region pattern correspondingto the layout of the core regions of optical waveguide elements of thecomponent,

C3) forming core regions using the core mask, a photolithographic and anetching process,

D) forming an upper cladding layer to cover the core region pattern andthe lower cladding layer, and

E) annealing in a controlled atmosphere.

The annealing step E) may e.g. be performed after the deposition of thecore layer and/or in connection with successive upper deposition andannealing steps.

In an embodiment of the present invention a clean and bare Silicon wafer(used as substrate, step A) is firstly oxidized (step B) to provide anoptical isolation layer (the ‘buffer layer’) of silica sufficientlythick that the magnitude of the evanescent field tail of the fieldpertaining to the waveguide cores is sufficiently low to ensurenegligible propagation loss. On top of the buffer layer a layer (termedthe ‘core layer’) of Si_(a)O_(x)N_(y)X_(z)H_(v)(RE)_(q)(TE)_(p) (withmeaning as described above) is deposited (step C1), containing one ormore dopants that effectively act to control the refractive index of thelayer, to make it optically active, and/or to adapt its thermalexpansion properties to those of the substrate. Depending upon themethod used to deposit the core layer a high temperature treatment(known as an anneal step) may be advantageous in order to stabilize theoptical and/or mechanical properties of the layer. The optical waveguidecircuitry is defined through standard optical lithography where aUV-transparent plate containing typically a chromium pattern replica ofthe waveguide design pattern (step C2) is pressed against a layer ofUV-sensitive polymer which has been spin coated onto the surface of thecore layer, subsequently the UV-sensitive polymer is exposed through themask and the pattern is developed (step C3). Following the exposure anddevelopment of the waveguide pattern into the polymer layer, the polymerpattern is used as masking material for dry etching (e.g. RIE—ReactiveIon Etching, ICP—Inductively Coupled Plasma) into the core layer (stepC3). In step C2), alternatively, a second mask system can be sandwichedbetween the core layer and the UV-sensitive polymer layer, which is usedto enhance selectivity and waveguide core profile. In an embodiment ofthe invention this second mask system may consist of oxide/polymer ornitride/polymer such as discussed for oxide polymers by J. M. Moran andD. Maydan: “High resolution, steep profile resist patterns” J. Vac. Sci.Technol., Vol. 16, No 6, November/December 1979 and for nitride polymersby H. Namatsu, Y. Ozaki, and K. Hirata, “high resolution trilevelresist”, J. Vac. Sci. Technol., 21(2), July/August 1982. In this way thedesign waveguide pattern is transferred into the core layer havingpredetermined cross-sectional properties as well as refractive index. Inorder to protect the thus defined waveguide core, and in order toenhance symmetry in the structure transverse to the direction ofpropagation, a layer of silica with optical properties as close to thoseof the buffer layer as the chosen fabrication technology permits isdeposited on top of the core structure (step D). The formation of thelatter layer (e.g. termed the upper cladding layer) may be formed usingsuccessive deposition and annealing steps (i.e. successive repetitionsof steps D and E). It may be advantageous to ensure that the uppercladding layer has a lower flow temperature than that of the core andlower cladding layers. This may be controlled by proper addition ofboron, phosphorus and/or fluorine (or any other dopants that reduces theflow temperature to the upper cladding layer.

A sample with a refractive index contrast of Δn/<n>=2.5% was madeaccording to the above mentioned procedure on top of 12 micron thermaloxide (SiO₂). Spiral waveguides with a cross section of 3×3 micron² wasfabricated and subsequently cladded with an optimized cladding procedure(a standard BPSG-cladding). The hereby obtained waveguides werecharacterized (as described in B. H. Larsen, et al., We1.2.6, ECOC-IOOC,2003) by direct, and hence, reliable propagation losses afterpropagation through up to 1 meter of waveguides. These characterizationexperiments reveal that when applying the parameters listed in EXAMPLE1, it is possible to remove the overtone of the N—H absorption peaklocated at 1508 nm as indicated by the curve ‘new process’ in FIG. 4illustrating optical absorption loss in dB/cm from λ=1500 nm to λ=1600nm for a waveguide manufactured according to different processingconditions, respectively, ‘with NH3’, ‘without NH3’ and a ‘new process’according to the invention.

By measuring the propagation loss as a function of the length at 1550 nmwe have managed to fabricate waveguides with an extremely low loss below0.03 dB/cm over a very broad range of wavelengths, as seen in FIG. 5showing optical propagation loss in dB at λ=1550 nm for a waveguideaccording to the invention as a function of waveguide length (in cm) andmode (TE or TM).

The sample made by the parameters listed under EXAMPLE 1 has beenanalyzed by Secondary Ion Mass Spectrometry (SIMS). Briefly the sampleunder investigation is sputtered by a flux of accelerated ions (hereCs⁺) by which material is continuously being removed from the surfacelayer. A simple monitoring and on-line analysis of fragments produced bysputtering will allow for chemical analysis as a function of sputteringtime. By calibration of sputtering yield and by comparison with relevantreferences, the intensity of relevant fragments can directly beconverted into concentrations (here e.g. N and P are relevant).

For the sample made by the procedure described in EXAMPLE 1 the N and Pconcentrations were determined to [N]=1.30·10²¹ atoms/cm³ and[P]=4.58·10²⁰ atoms/cm³ corresponding to an [N]/[P] ratio of 2.83.

Assuming a density close to 2.3 g/cm³ as has been reported for SiON-typematerials (cf. “Plasma-enhanced growth, composition and refractive indexof silicon oxy-nitride films”, K. E. Mattsson, J. Appl. Phys. 77, No.12, p. 6616-6623, 1995) and a molar weight close to that of SiO₂ (28.086g/mole+2·15.999 g/mole=60.0840 g/mole) one can convert the measuredatomic densities to stoichiometric values since the density of a SiONtype material is close to 2.3 g/cm³/60.0840 g/mole=0.0383 mole/cm³.Multiplying with Avogadro's constant N_(a) one obtain an atom density of2.3052·10²² atoms/cm³.

This leads to the following relative concentrations y and z of nitrogenand phosphorus, respectively:N _(y) : y=1.30·1021/2.3052·1022=0.056P _(z) : z=4.58·1020/2.0352·1022=0.020

EXAMPLE2

A sample comprising an optical waveguide according to the invention wasmade as described in example 1 with the only difference that the 5%PH₃/95% N₂ gas flow was increased from 10 to 50 sccm. The hereby grownPECVD films delaminated upon annealing due to the high P-content. Thus,there is an upper limited to the amount of PH₃ which can be presentunder PECVD growth of a core under the above mentioned processparameters.

EXAMPLE 3

A sample comprising an optical waveguide according to the invention wasmade as described in example 1. The structure of the resultingwaveguides were subsequently analyzed by Scanning Electron Microscopy(SEM) of polished cross sectional cuts. FIG. 7 a shows the resultingwaveguide profiles for an isolated waveguide 100 comprising core 33,lower 61 and upper 62 cladding regions. From FIG. 7 a, it is evidentthat the waveguide core 33 (having a width of app. 7 μm as indicated inthe SEM-photo) is (partially) surrounded by the upper cladding layer 62,and furthermore, no defects can be seen close to the waveguide coreregion. For closer spaced waveguides (e.g. for edge-to-edge spacings 72less than 4 μm, cf. FIG. 7 b), one observes an apparent reaction betweenthe (upper) cladding layer 62 and the waveguide core material 33resulting in the nucleation and growth of small crystallites/particles71 next to the waveguide core regions. FIG. 7 b shows a representativeSEM image of this particle formation process in-between neighboringwaveguide core regions (here having an edge to edge spacing of 5 μm asindicated in the SEM-photo). It has surprisingly turned out that theformation of these crystallites can be prevented by adding a bufferlayer between the core region and the upper cladding layer. In thepresent case a buffer layer of 0.35 μm undoped PECVD SiO₂ on top of theetched core layer was enough to avoid crystallite/particle formation.The appropriate minimum thickness depends on the dopant levels of thecore and cladding, respectively (higher dopant levels=>higherthickness). The buffer layer thickness is preferably in the range from0.2 μm to 1.0 μm. Other buffers/barriers such as PECVD BPSG withalternative B/P doping levels, SiON, Si_(x)N_(y) (e.g. Si₃N₄), etc. (andcorrespondingly optimized layer thicknesses) might be applied.

In a preferred embodiment of the method outlined in Example 1, asub-step “C4) of forming a barrier layer on top of said core regionpattern, and optionally on top of the lower cladding layer not coveredby the core region pattern;”

is inserted before step D) of forming an upper cladding layer to coverthe core region pattern and the lower cladding layer. Optionally, abarrier layer may be applied below the core region pattern by insertinga sub-step C0) of forming a barrier layer on top of said lower claddinglayer. The latter has the advantage of fully isolating the core layerfrom the lower and upper cladding layers. An optional annealing step maybe inserted after the barrier layer formation step(s) to relax thebarrier layer. An advantage of inserting the buffer layer or layers isthat the tendency to formation of crystallites may be lowered oreliminated. Thereby the out-diffusion of Phosphorus from the core regionmay be lowered or eliminated. It may further have the advantage ofreducing stress in the waveguide core. These effects of the inclusion ofa barrier layer between the core and cladding regions are particularlyadvantageous for waveguides according to the present invention for whichthe concentration (y) of X (including e.g. P) is larger than theconcentration (z) of N (i.e. for y/z>1 as defined by the presentinvention).

It might be tempting to associate the observed particles due to theformation of B₂O₃, P₂O₅ or BPO₄, crystallites. Formation of suchparticles has been observed as a consequence of Boron and Phosphorussegregation during annealing of BPSG films (see e.g. S. Imai et al.,Appl. Phys. Lett. 60(22), p. 1761). Thus, adding the above mentionedbuffer will clearly alter the relative concentration of B and Ppreventing the local nucleation and growth of crystallites.

EXAMPLE 4

A core made according to Example 1 has been made with two different PH₃flows. The index was in both cases tuned by adjusting the N₂O flowkeeping everything else constant. From FIG. 8, it is evident that it ispossible to bridge an index range (measured at 1550 nm) fromapproximately 1.44 up to 1.5 for both series of PH₃ flows. From thefigure it is interesting to note that the birefringence (n(TE)−n(TM)) islowest for the 5 sccm PH₃ series 81 as compared to the 15 sccm PH₃series 82. Furthermore, for the 15 sccm series it is observed that thebirefringence increases with increasing refractive index whereas itstays approximately constant for the 5 sccm PH₃ series at a lower valueof −2·10⁻³ to −3·10⁻³.

Thus the exact PH₃ flow value can be used as an additional stressoptimization parameter when tuning the exact core process in connectionwith further applications of this type of core.

BASIC ELEMENTS

For waveguides according to the invention comprising material of thecomposition Si_(a)O_(x)N_(y)X_(z)H_(v), the individual elements may beintroduced—mainly from a vapour phase—from the following compounds:

Silicon, Si:

SiH₄, SiF₄, SiCl₄, SiF₄, Si₂H₆, SiH₂Cl₂, SiCl₂F₂, SiH₂F₂ or any othersilicon containing gases or solids involving the use of hydrogen,chlorine, oxygen or even from solid compounds such as SiOx or spin ontype glasses as well as sol-gel compounds containing Si.

Oxygen, O:

N₂O, NO, N₂, NO₂, O₂, H₂O; H₂O₂, CO, CO₂

Nitrogen, N:

N₂O, NO, N₂, NO₂, NH₃, N₂

Boron, B:

B₂H₆ or from solid compounds such as B₂O₃

Aluminum, Al:

AlH₃ or liquid solved Organo-Al complexes

Phosphorus, P:

PH₃

Sulfur, S:

H₂S, SO, SO₂

Germanium, Ge:

GeH₄ or solid compounds such as GeO₂

Arsenic, As:

AsH₃

Antimony, Sb:

Sb dissolved in Organo compounds.

Carrier gas selected from:

N₂, He, Ne, Ar, Kr, Xe

AN EXAMPLE BASED ON ATOMIC CONCENTRATIONS

In the following a correlation is established between relativestoichiometric concentrations (the a, x, y, z, v's inSi_(a)O_(x)N_(y)X_(z)H_(v)) and corresponding atomic densities forvarious mass densities of the resulting material. This is e.g. of usewhen a SIMS measurement is made to determine the atomic concentration ofa given sample.

The ‘atomic’ concentration (units Si_(a)O_(x)N_(y)X_(z)H_(v)/cm³) in agiven volume of a Si_(a)O_(x)N_(y)X_(z)H_(v) type material is calculatedas the mass density (ρ(g/cm³)) divided by the mole mass M_(tot) (g/mole)multiplied by Avogadro's number N_(a). The total mole mass ofSi_(a)O_(x)N_(y)X_(z)H_(v) is given by a weighted sum of the mole massesof the constituting elements, i.e.M_(tot)=a·M_(Si)+x·M_(o)+y·M_(N)+z·M_(X)+v·M_(H).

The individual atom densities N_(at) (atoms/cm³) of each type of atomwill then be given by:N _(at)(Si)=a/(a+x+y+z+v)·total number of atoms=a·ρ·N_(a)/[(a+x+y+z+v)·M _(tot)]N _(at)(O)=x/(a+x+y+z+v)·total number of atoms=x·ρ·N _(a)/[(a+x+y+z+v)·M_(tot)]N _(at)(N)=y/(a+x+y+z+v)·total number of atoms=y·ρ·N _(a)/[(a+x+y+z+v)·M_(tot)]N _(at)(X)=z/(a+x+y+z+v)·total number of atoms=z·ρ·N _(a)/[(a+x+y+z+v)·M_(tot)]N _(at)(H)=v/(a+x+y+z+v)·total number of atoms=v·ρ·N _(a)/[(a+x+y+z+v)·M_(tot)]

In an embodiment of the invention the above method can be illustrated byassuming a type of structure such as Si_((1−z))O_((2−y))N_(y)P_(z)wherein P is taken as an example from the group of elements denoted by Xin the general formula (‘X’=B, Al, P, S, As, Sb). In this example, P isassumed to substitute Si and N to substitute O. The addition of a smallamount of H_(v) (either intentionally or unintentionally) is notexpected to have a major impact on either ρ or M_(tot) due the smallsize and mass of hydrogen as well as the small concentration of Hexpected. However, the local presence of H might be important since thematerial should be valence neutral, i.e. (1−z)·[oxidation state ofSi]+(2−y)·[oxidation state of O]+y·[oxidation state of N]+z·[oxidationstate of P]+v·[oxidation state of H]=0.

The reported oxidation states are:

H:

+1 and −1 with +1 being the most obvious configuration in connectionwith covalent hydrides

O:

−2 and −1, with −2 being the most obvious state

Si:

+4, +6, −2, with +4 being the most stable configuration in connectionwith SiO₂ like materials

N:

+3, −3, +4, +5, with the −3 state being the most stable configuration inthe present content

P:

+3, +5, −3, with +5 being the most stable state.

Thus one can write:

(1−z)·(+4)+(2−y)·(−2)+y·(−3)+z·(+5)+v·(+1)=4−4z−4+2y−3y+5z+v=z−y+v=0,i.e. z=y−v, assuming the above mentioned oxidation states of theindividual compounds. Thus in this simple model, a small amount ofhydrogen will allow for z<y, i.e. for more nitrogen than phosphors.Alternative valance states of the constituents may not be neglected.

Assuming a Si_((1−z))O_((2−y))N_(y)P_(z) type of material, z will bebetween 0 and 1 and y will be between 0 and 2. In an embodiment of theinvention, z will be between e.g. 0 and 0.2 and y will be between 0 and0.1 and y>z. The presence of hydrogen is not incorporated in thefollowing calculations although a small amount may be present.

In the above mentioned intervals, M_(tot) has been calculated as:

M_(tot)=(1−z)·M_(Si)+(2−y)·M_(o)+y·M_(N)+z·M_(p), with

M_(Si)=28.086 g/mole

M_(o)=15.999 g/mole

M_(P)=30.974 g/mole

M_(N)=14.007 g/mole

The result for M_(tot) has been summarized in table 1. As can be seenfrom table 1, M_(tot) is only varying between 59.69 g/mole and 60.17g/mole. TABLE 1 Mole mass (g/mole) for a Si_((1-z))O_((2-y))N_(y)P_(z)type material with z ε [0, 0.1], y ε [0, 0.2] and y > z. mole mass(g/mole) y z 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11 060.08 60.06 60.04 60.02 60.00 59.98 59.96 59.94 59.92 59.90 59.88 59.860.01 60.09 60.07 60.05 60.03 60.01 59.99 59.97 59.95 59.93 59.91 59.890.02 60.10 60.08 60.06 60.04 60.02 60.00 59.98 59.96 59.94 59.92 0.0360.11 60.09 60.07 60.05 60.03 60.01 59.99 59.97 59.95 0.04 60.12 60.1060.08 60.06 60.04 60.02 60.00 59.98 0.05 60.13 60.11 60.09 60.07 60.0560.03 60.01 0.06 60.14 60.12 60.10 60.08 60.06 60.04 0.07 60.15 60.1360.11 60.09 60.07 0.08 60.16 60.14 60.12 60.10 0.09 60.16 60.14 60.120.1 60.17 60.15 mole mass (g/mole) y z 0.12 0.13 0.14 0.15 0.16 0.170.18 0.19 0.2 0 59.84 59.83 59.81 59.79 59.77 59.75 59.73 59.71 59.690.01 59.87 59.85 59.83 59.81 59.79 59.77 59.75 59.73 59.71 0.02 59.9059.88 59.86 59.84 59.82 59.80 59.78 59.76 59.74 0.03 59.93 59.91 59.8959.87 59.85 59.83 59.81 59.79 59.77 0.04 59.96 59.94 59.92 59.90 59.8859.86 59.84 59.82 59.80 0.05 59.99 59.97 59.95 59.93 59.91 59.89 59.8759.85 59.83 0.06 60.02 60.00 59.98 59.96 59.94 59.92 59.90 59.88 59.860.07 60.05 60.03 60.01 59.99 59.97 59.95 59.93 59.91 59.89 0.08 60.0860.06 60.04 60.02 60.00 59.98 59.96 59.94 59.92 0.09 60.10 60.08 60.0760.05 60.03 80.01 59.99 59.97 59.95 0.1 60.13 60.11 60.09 60.07 60.0560.03 60.01 59.99 59.97

Applying the above mentioned intervals, it is possible to calculate theatomic densities of Si, O, P and N when knowing the mass density(ρ(g/cm³)) of the material. However, the mass density is expected tovary with the atomic structure. For a PECVD SiON-type material, adensity between 2.26 g/cm³ and 2.35 g/mole has been measured (cf. KentErik Mattsson, Ph.D thesis MIC, The Technical University of Denmark,1994). For various forms of quarts, densities such as 2.32, 2.19, 2.26,2.635, 2.13 g/mole can be found in “Handbook of chemistry and Physics”66^(TH) edition.

As an illustrative example, the mass density is assumed to be 2.3 g/molefor a Si_((1−z))O_((2−y))N_(y)P_(z) type material with zε[0, 0.1], yε[0,0.2] and y>z. The corresponding atomic densities for Si, O, P, and N canbe calculated. The resulting values are shown in tables 2, 3, 4, and 5,respectively, wherein 10²¹ is written as 1E21, i.e. e.g. 7.7·10²¹ iswritten as 7.7E21. TABLE 2 Atomic density of Si (atoms/cm³) for aSi_((1-z))O_((2-y))N_(y)P_(z) type material with z ε [0, 0.1], y ε [0,0.2] and y > z. The mass density is assumed to be 2.3 g/mole. Density ofSi atoms/cm3 y z 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.110 7.7E21 7.7E21 7.7E21 7.7E21 7.7E21 7.7E21 7.7E21 7.7E21 7.7E21 7.7E217.7E21 7.7E21 0.01 7.6E21 7.6E21 7.6E21 7.6E21 7.6E21 7.6E21 7.6E217.6E21 7.6E21 7.6E21 7.6E21 0.02 7.5E21 7.5E21 7.5E21 7.5E21 7.5E217.5E21 7.5E21 7.5E21 7.5E21 7.6E21 0.03 7.5E21 7.5E21 7.5E21 7.5E217.5E21 7.5E21 7.5E21 7.5E21 7.5E21 0.04 7.4E21 7.4E21 7.4E21 7.4E217.4E21 7.4E21 7.4E21 7.4E21 0.05 7.3E21 7.3E21 7.3E21 7.3E21 7.3E217.3E21 7.3E21 0.06 7.2E21 7.2E21 7.2E21 7.2E21 7.2E21 7.2E21 0.07 7.1E217.1E21 7.1E21 7.1E21 7.1E21 0.08 7.1E21 7.1E21 7.1E21 7.1E21 0.09 7.0E217.0E21 7.0E21 0.1 6.9E21 6.9E21 Density of Si atoms/cm3 y z 0.12 0.130.14 0.15 0.16 0.17 0.18 0.19 0.2 0 7.7E21 7.7E21 7.7E21 7.7E21 7.7E217.7E21 7.7E21 7.7E21 7.7E21 0.01 7.6E21 7.6E21 7.6E21 7.6E21 7.6E217.6E21 7.6E21 7.7E21 7.7E21 0.02 7.6E21 7.6E21 7.6E21 7.6E21 7.6E217.6E21 7.6E21 7.6E21 7.6E21 0.03 7.5E21 7.5E21 7.5E21 7.5E21 7.5E217.5E21 7.5E21 7.5E21 7.5E21 0.04 7.4E21 7.4E21 7.4E21 7.4E21 7.4E217.4E21 7.4E21 7.4E21 7.4E21 0.05 7.3E21 7.3E21 7.3E21 7.3E21 7.3E217.3E21 7.3E21 7.3E21 7.3E21 0.06 7.2E21 7.2E21 7.2E21 7.2E21 7.2E217.2E21 7.2E21 7.2E21 7.3E21 0.07 7.2E21 7.2E21 7.2E21 7.2E21 7.2E217.2E21 7.2E21 7.2E21 7.2E21 0.08 7.1E21 7.1E21 7.1E21 7.1E21 7.1E217.1E21 7.1E21 7.1E21 7.1E21 0.09 7.0E21 7.0E21 7.1E21 7.0E21 7.0E217.0E21 7.0E21 7.0E21 7.0E21 0.1 6.9E21 6.9E21 6.9E21 6.9E21 6.9E216.9E21 6.9E21 6.9E21 6.9E21

From table 2, it is seen that the density of Si is between 6.9E21 and7.7E21 atoms/cm³, assuming ρ=2.3 g/cm³. TABLE 3 Atomic density of N(atoms/cm³) for a Si_((1-z))O_((2-y))N_(y)P_(z) type material with z ε[0, 0.1], y ε [0, 0.2] and y > z. The mass density is assumed to be 2.3g/mole. Density of N atoms/cm3 y z 0 0.01 0.02 0.03 0.04 0.05 0.06 0.070.08 0.09 0.1 0.11 0 0.0E+00 7.7E19 1.5E20 2.3E20 3.1E20 3.8E20 4.6E205.4E20 6.2E20 6.9E20 7.7E20 8.5E20 0.01 7.7E19 1.5E20 2.3E20 3.1E203.8E20 4.6E20 5.4E20 6.2E20 6.9E20 7.7E20 8.5E20 0.02 1.5E20 2.3E203.1E20 3.8E20 4.6E20 5.4E20 6.2E20 6.9E20 7.7E20 8.5E20 0.03 2.3E203.1E20 3.8E20 4.6E20 5.4E20 6.2E20 6.9E20 7.7E20 8.5E20 0.04 3.1E203.8E20 4.6E20 5.4E20 6.2E20 6.9E20 7.7E20 8.5E20 0.05 3.8E20 4.6E205.4E20 6.1E20 6.9E20 7.7E20 8.5E20 0.06 4.6E20 5.4E20 6.1E20 6.9E207.7E20 8.5E20 0.07 5.4E20 6.1E20 6.9E20 7.7E20 8.5E20 0.08 6.1E20 6.9E207.7E20 8.5E20 0.09 6.9E20 7.7E20 8.4E20 0.1 7.7E20 8.4E20 Density of Natoms/cm3 y z 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0 9.3E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.01 9.3E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.02 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.03 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.04 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.05 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.06 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.07 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.08 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.09 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21 0.1 9.2E201.0E21 1.1E21 1.2E21 1.2E21 1.3E21 1.4E21 1.5E21 1.5E21

From table 3, it is seen that the density of N is between 0 and 1.5E21atoms/cm³, assuming ρ=2.3 g/cm³. TABLE 4 Atomic density of P (atoms/cm³)for a Si_((1-z))O_((2-y))N_(y)P_(z) type material with z ε [0, 0.1], y ε[0, 0.2] and y > z. The mass density is assumed to be 2.3 g/mole.Density of P atoms/cm3 y z 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080.09 0.1 0.11 0 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E000.0E00 0.0E00 0.0E00 0.0E00 0.01 7.7E19 7.7E19 7.7E19 7.7E19 7.7E197.7E19 7.7E19 7.7E19 7.7E19 7.7E19 7.7E19 0.02 1.5E20 1.5E20 1.5E201.5E20 1.5E20 1.5E20 1.5E20 1.5E20 1.5E20 1.5E20 0.03 2.3E20 2.3E202.3E20 2.3E20 2.3E20 2.3E20 2.3E20 2.3E20 2.3E20 0.04 3.1E20 3.1E203.1E20 3.1E20 3.1E20 3.1E20 3.1E20 3.1E20 0.05 3.8E20 3.8E20 3.8E203.8E20 3.8E20 3.8E20 3.8E20 0.06 4.6E20 4.6E20 4.6E20 4.6E20 4.6E204.6E20 0.07 5.4E20 5.4E20 5.4E20 5.4E20 5.4E20 0.08 6.1E20 6.1E20 6.1E206.1E20 0.09 6.9E20 6.9E20 6.9E20 0.1 7.7E20 7.7E20 Density of Patoms/cm3 y z 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0 0.0E000.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.0E00 0.01 7.7E197.7E19 7.7E19 7.7E19 7.7E19 7.7E19 7.7E19 7.7E19 7.7E19 0.02 1.5E201.5E20 1.5E20 1.5E20 1.5E20 1.5E20 1.5E20 1.5E20 1.5E20 0.03 2.3E202.3E20 2.3E20 2.3E20 2.3E20 2.3E20 2.3E20 2.3E20 2.3E20 0.04 3.1E203.1E20 3.1E20 3.1E20 3.1E20 3.1E20 3.1E20 3.1E20 3.1E20 0.05 3.8E203.8E20 3.9E20 3.9E20 3.9E20 3.9E20 3.9E20 3.9E20 3.9E20 0.06 4.6E204.6E20 4.6E20 4.6E20 4.6E20 4.6E20 4.6E20 4.6E20 4.6E20 0.07 5.4E205.4E20 5.4E20 5.4E20 5.4E20 5.4E20 5.4E20 5.4E20 5.4E20 0.08 6.1E206.2E20 6.2E20 6.2E20 6.2E20 6.2E20 6.2E20 6.2E20 6.2E20 0.09 6.9E206.9E20 6.9E20 6.9E20 6.9E20 6.9E20 6.9E20 6.9E20 6.9E20 0.1 7.7E207.7E20 7.7E20 7.7E20 7.7E20 7.7E20 7.7E20 7.7E20 7.7E20

From table 4, it is seen that the density of P is between 0 and 7.7E20atoms/cm³, assuming ρ=2.3 g/cm³. TABLE 5 Atomic density of O (atoms/cm³)for a Si_((1-z))O_((2-y))N_(y)P_(z) type material with z ε [0, 0.1], y ε[0, 0.2] and y > z. The mass density is assumed to be 2.3 g/mole.Density of O atioms/cm3 y z 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.080.09 0.1 0.11 0 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 0.01 1.5E22 1.5E22 1.5E22 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 0.02 1.5E22 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 0.03 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 0.04 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 0.05 1.5E22 1.5E22 1.5E221.5E22 1.5E22 1.5E22 1.5E22 0.06 1.5E22 1.5E22 1.5E22 1.5E22 1.5E221.5E22 0.07 1.5E22 1.5E22 1.5E22 1.5E22 1.5E22 0.08 1.5E22 1.5E22 1.5E221.5E22 0.09 1.5E22 1.5E22 1.5E22 0.1 1.5E22 1.5E22 Density of Oatioms/cm3 y z 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.2 0 1.5E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.01 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.02 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.03 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.04 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.05 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.06 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.07 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.08 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.09 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 0.1 1.4E221.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22 1.4E22

From table 5, it is seen that the density of O is between 1.4E22 and1.5E22 atoms/cm³, assuming ρ=2.3 g/cm³.

Similar evaluations can be performed for a range of mass densitiesaround 2.3 g/mole. The hereby calculated maximal and minimal atomicdensities have been summarized in table 6. TABLE 6 Mass densitiesbetween 1.5 and 4 g/mole and the hereby maximum and minimum number ofatoms per cm³ assuming a Si_((1-z))O_((2-y))N_(y)P_(z) type materialwith z ε [0, 0.1], y ε [0, 0.2] and y > z. Massefylde Si atoms/cm³ Oatoms/cm³ N atoms/cm³ P atoms/cm³ g/cm³ min maks min maks min maks minmaks 1.5 4.5E21 5.0E21 9.0E21 1.0E22 0.0E00 1.0E21 0.0E00 5.0E20 1.64.8E21 5.4E21 9.6E21 1.1E22 0.0E00 1.1E21 0.0E00 5.4E20 1.7 5.1E215.7E21 1.0E22 1.1E22 0.0E00 1.1E21 0.0E00 5.7E20 1.8 5.4E21 6.1E211.1E22 1.2E22 0.0E00 1.2E21 0.0E00 6.0E20 1.9 5.7E21 6.4E21 1.1E221.3E22 0.0E00 1.3E21 0.0E00 6.4E20 2 6.9E21 7.7E21 1.4E22 1.5E22 0.0E001.5E21 0.0E00 7.7E20 2.1 6.3E21 7.1E21 1.3E22 1.4E22 0.0E00 1.4E210.0E00 7.0E20 2.2 6.6E21 7.4E21 1.3E22 1.5E22 0.0E00 1.5E21 0.0E007.4E20 2.3 6.9E21 7.7E21 1.4E22 1.5E22 0.0E00 1.5E21 0.0E00 7.7E20 2.47.2E21 8.1E21 1.4E22 1.6E22 0.0E00 1.6E21 0.0E00 8.0E20 2.5 7.5E218.4E21 1.5E22 1.7E22 0.0E00 1.7E21 0.0E00 8.4E20 2.6 7.8E21 8.7E211.6E22 1.7E22 0.0E00 1.7E21 0.0E00 8.7E20 2.7 8.1E21 9.1E21 1.6E221.8E22 0.0E00 1.8E21 0.0E00 9.0E20 2.8 8.4E21 9.4E21 1.7E22 1.9E220.0E00 1.9E21 0.0E00 9.4E20 2.9 8.7E21 9.8E21 1.7E22 1.9E22 0.0E002.0E21 0.0E00 9.7E20 3 9.0E21 1.0E22 1.8E22 2.0E22 0.0E00 2.0E21 0.0E001.0E21 3.1 9.3E21 1.0E22 1.9E22 2.1E22 0.0E00 2.1E21 0.0E00 1.0E21 3.29.6E21 1.1E22 1.9E22 2.1E22 0.0E00 2.2E21 0.0E00 1.1E21 3.3 9.9E211.1E22 2.0E22 2.2E22 0.0E00 2.2E21 0.0E00 1.1E21 3.4 1.0E22 1.1E222.0E22 2.3E22 0.0E00 2.3E21 0.0E00 1.1E21 3.5 1.0E22 1.2E22 2.1E222.3E22 0.0E00 2.4E21 0.0E00 1.2E21 3.6 1.1E22 1.2E22 2.2E22 2.4E220.0E00 2.4E21 0.0E00 1.2E21 3.7 1.1E22 1.2E22 2.2E22 2.5E22 0.0E002.5E21 0.0E00 1.2E21 3.8 1.1E22 1.3E22 2.3E22 2.5E22 0.0E00 2.6E210.0E00 1.3E21 3.9 1.2E22 1.3E22 2.3E22 2.6E22 0.0E00 2.6E21 0.0E001.3E21 4 1.2E22 1.3E22 2.4E22 2.7E22 0.0E00 2.7E21 0.0E00 1.3E21

Based on table 6 one can state that for a Si_((1−z))O_((2−y))N_(y)P_(z)type material with z ε[0, 0.1], yε[0, 0.2] and y>z with a mass densitybetween 1.5 and 4 g/cm³ the atomic densities of Si, O, N and P will bein the ranges indicated below:

The atomic density of Si is between 4.5E21 and 1.3E22 atoms/cm³.

The atomic density of O is between 9.0E21 and 2.7E22 atoms/cm³.

The atomic density of N is between 0 and 2.7E21 atoms/cm³.

The atomic density of P is between 0 and 1.3E21 atoms/cm³.

In an embodiment of the invention (EXAMPLE 1), the density of N and Pwas determined to [N] =1.30E21 atoms/cm³ and [P] =4.58E20 atoms/cm³which is within the above mentioned ranges for N and P.

FIG. 6 shows a schematic (x-y-plane) cross sectional view of an opticalcomponent 100 according to the invention comprising a base (or lowercladding) layer 61 formed on a substrate 10 with various waveguide coreelements 31, 32, 33 applied to the base layer and covered by an uppercladding layer 62 (the combined cladding layers 61, 62 being denoted 6in FIG. 6). The upper cladding layer has an upper surface 621, possiblybeing corrugated (although not to scale in FIG. 6) due to an anneal andreflow procedure. Waveguides 31, 32, 33 of different widths w₁, w₂, w₃,respectively, and identical height h (equal to the thickness of the corelayer) are shown. The waveguides have end facets 331 (assuming that thecross section of FIG. 6 is an ‘end view’ of a component). The substrate(e.g. a silicon substrate) 10 has a bottom essentially planar face 11(x-z-plane).

The invention is defined by the features of the independent claim(s).Preferred embodiments are defined in the dependent claims.

Some preferred embodiments have been shown in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims.

1. An optical waveguide for guiding light in a predefined wavelengthrange, the optical waveguide comprising core and cladding regions forconfining light, the core and/or cladding region or regions being formedon a substrate, and the whole or a part of the core and/or claddingregion or regions comprising material of the stoichiometric compositionSi_(a)O_(x)N_(y)X_(z)H_(v), wherein a is in the range from 0.1 to 3.5; xis in the range from 0 to 2.5; y is in the range from 3.9 to 4.1 or inthe range from 0.02 to 0.3; z is in the range from 0 to 0.3 and X isselected from the group of elements B, Al, P, S, As, Sb and combinationsthereof, and the ratio y/z is larger than 1.2, such as larger than 1.5,such as larger than 1.8, such as larger than 2.0, such as larger than2.5, such as larger than 3.0, such as larger than 3.5, such as largerthan 4.0, such as larger than 4.5, such as larger than 5.0, such aslarger than 5.5, such as larger than 6.0, such as larger than 7.0, suchas larger than 8.0.
 2. An optical waveguide according to claim 1 whereinthe ratio y/z is in the range from 1.2 to 100, such as 1.2 to 20, suchas 1.2 to 10, such as 1.5 to 8.0, such as 2.0 to 4.0, such as 2.5 to3.5.
 3. An optical waveguide according to claim 1 wherein the number adefining the relative concentration of the element Si is in the rangefrom 0.9 to 1.1 or in the range from 2.9 to 3.1.
 4. An optical waveguideaccording to claim 1 wherein the number x defining the relativeconcentration of the element O is in the range from 1.9 to 2.1 or in therange from 0 to 0.1.
 5. An optical waveguide according to claim 1wherein the number y defining the relative concentration of the elementN is in the range from 3.9 to 4.1 or in the range from 0.03 to 0.2, suchas in the range from 0.04 to 0.10.
 6. An optical waveguide according toclaim 1 wherein the number z defining the relative concentration of theelement X selected from the group comprising B, Al, P, S, As, Sb andcombinations thereof is in the range from 0.005 to 0.2, such as in therange from 0.01 to 0.10.
 7. An optical waveguide according to claim 1wherein a is in the range from 0.8 to 1.2 and x is in the range from 1.8to 2.2 and y is in the range from 0.01 to 0.5 and z is in the range from0.005 to 0.2.
 8. An optical waveguide according to claim 1 wherein a isin the range from 2.8 to 3.2 and y is in the range from 3.8 to 4.2 and xis in the range from 0.01 to 0.5 and z is in the range from 0.005 to0.2.
 9. An optical waveguide according to claim 1 wherein the number adefining the relative concentration of the element Si is in the rangefrom 0.9 to 1.1, the number x defining the relative concentration of theelement O is in the range from 1.9 to 2.1, the number y defining therelative concentration of the element N is in the range from 0.015 to0.12, and the number z defining the relative concentration of theelement X is in the range from 0.005 to 0.04.
 10. An optical waveguideaccording to claim 1 wherein the optical absorption peak at λ=1508 nmdue to Si:N—H bonds is smaller than 0.1 dB/cm, such as smaller than 0.05dB/cm such as smaller than 0.01 dB/cm.
 11. An optical waveguideaccording to claim 1 wherein the number v defining the relativeconcentration of the element H is such that the relative concentrationv/(a+x+y+z+v) of H in Si_(a)O_(x)N_(y)X_(z)H_(v) is smaller than 10⁻²,such as smaller than 10⁻³, such as smaller than 10⁻⁴, such as smallerthan 10⁻⁵.
 12. An optical waveguide according to claim 1 wherein theatomic concentration of hydrogen is larger than the atomic concentrationof nitrogen and/or phosphorus.
 13. An optical waveguide according toclaim 1 wherein the atomic concentration of hydrogen is larger than 5at. %.
 14. An optical waveguide according to claim 1 wherein the numberv defining the relative concentration of the element H is such that theconcentration v/y of H relative to N is smaller than 10⁻², such assmaller than 10⁻³, such as smaller than 10⁻⁴.
 15. An optical waveguideaccording to claim 1 wherein the number v defining the relativeconcentration of the element H is such that the concentration v/z of Hrelative to X is smaller than 10⁻², such as smaller than 10⁻³, such assmaller than 10⁻⁴, X being an element selected from the group comprisingB, Al, P, S, As, Sb and combinations thereof.
 16. An optical waveguideaccording to claim 1 wherein the element or elements X or the materialSi_(a)O_(x)N_(y)X_(z)H_(v) comprises at least 50% phosphorus such as atleast 75% phosphorus such as at least 90% phosphorus, such as 100%phosphorus.
 17. An optical waveguide according to claim 1 wherein theelement or elements X or the material Si_(a)O_(x)N_(y)X_(z)H_(v)comprises at least two elements X(1), X(2), . . . , X(n) where n≦7,selected from the group comprising B, Al, P, S, Ge, As, Sb of relativeconcentration z₁, z₂, . . . , z_(n), respectively, where z=z₁+z₂+z₃+ . .. +z_(n) and wherein z₁/z is larger than 0.50 such as larger than 0.75such as larger than 0.90.
 18. An optical waveguide according to claim 17wherein n=2 and X(1) is P and X(2) is B or Ge.
 19. An optical waveguideaccording to claim 17 wherein n=3 and X(1) is P, X(2) is B and X(3) isGe.
 20. An optical waveguide according to claim 1 wherein the waveguidecore and/or cladding layers comprise material of the stoichiometriccomposition Si_((1−z))O_((2−y))N_(y)X_(z) wherein X is an element formthe group comprising B, Al, P, S, As, Sb or a combination thereof. 21.An optical waveguide according to claim 20 wherein X is P.
 22. Anoptical waveguide according to claim 20 wherein 0≦y<0.2 and 0≦z<0.1. 23.An optical waveguide according to claim 1 wherein the atomic density ofsilicon N_(at)(Si) is in the range 4.5·10²¹<N_(at)(Si)<1.3·10²², such asin the range 5.1·10²¹<N_(at)(Si)<9.1·10²¹, the atomic density of oxygenN_(at)(O) is in the range 9.0·10²¹<N_(at)(O)<2.7·10²², such as in therange 1.0·10²²<N_(at)(O)<1.8·10²², the atomic density of nitrogenN_(at)(N) is in the range 0<N_(at)(N)<2.7·10²¹, such as in the range0<N_(at)(N)<1.8·10²¹, and the atomic density of phosphorus N_(at)(P) isin the range 0<N_(at)(P)<1.3·10²¹, such as in the range0<N_(at)(P)<9.0·10²⁰.
 24. An optical waveguide according to claim 1wherein the core and/or cladding region comprises material having arefractive index at a wavelength of 1550 nm in the range 1.45-2.02, suchas in the range from 1.45 to 1.60, such as in the range from 1.48 to1.56.
 25. An optical waveguide according to claim 1 wherein the opticalwaveguide is adapted to guide light in a wavelength range from 250 nm to3.6 μm, such as in the range from 850 nm to 1800 nm.
 26. An opticalwaveguide according to claim 1 wherein the optical waveguide is adaptedto guide light comprising wavelengths in the range from 1260 nm to 1660nm, such as in the range 1530-1565 nm, or in the range 1460-1530 nm, orin the range 1360-1460 nm, or in the range 1260-1360 nm.
 27. An opticalwaveguide according to claim 1 wherein the waveguide core and/orcladding further comprises a rare earth elements selected from the groupof elements comprising Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu or combinations thereof.
 28. An optical waveguide according toclaim 1 wherein one or more of the rare earth elements are present inmolar concentrations in the range from 50 to 5000 ppm mole/mole.
 29. Anoptical waveguide according to claim 1 wherein the core and/or claddingregion further comprises one or more TE-dopant elements for controllingthe thermal expansion of the waveguide.
 30. An optical waveguideaccording to claim 1 wherein the thermal expansion of one or more of thelayers constituting the core and cladding regions of the waveguideis/are adapted to the thermal expansion of the substrate by adding oneor more TE-dopant elements to said one or more layers of the waveguide.31. An optical waveguide according to claim 29 wherein said TE-dopantelement or elements are selected from the group of elements comprisingAl, F, Ti, or combinations thereof.
 32. An optical waveguide accordingto claim 29 wherein said TE-dopant element or elements are present inthe core/and or cladding region or regions in molar concentrations inthe range from 0 to 5%.
 33. An optical waveguide according to claim 29wherein said dopant element or elements are present in the core/and orcladding region or regions in amounts sufficient to provide acoefficient of thermal expansion between 1×10⁻⁷ ° C.⁻¹ and 15×10⁻⁷ °C.⁻¹.
 34. An optical waveguide according to claim 1 comprising a buffermaterial constituting a barrier between the core and cladding regionsand fully or partially surrounding said core region.
 35. An opticalwaveguide according to claim 34 wherein said buffer material is selectedfrom the group SiO₂, Si_(x)N_(y), such as Si₃N₄, PECVD BPSG withalternative B/P doping levels and combinations thereof.
 36. An opticalwaveguide according to claim 1 wherein said material further comprisesGe.
 37. A method of manufacturing an optical waveguide according toclaim 1, the method comprising the steps of A) providing a substrate, B)forming a lower cladding layer on the substrate, C) forming a coreregion of said optical waveguide on the lower cladding layer, D) formingan upper cladding layer to cover the core region and the lower claddinglayer.
 38. A method according to claim 37 wherein step C) comprises thesub-steps C1) forming a core layer on the lower cladding layer, C2)providing a core mask comprising a core region pattern corresponding tothe layout of the core region of said optical waveguide, and C3) formingcore regions using the core mask, a photolithographic and an etchingprocess.
 39. A method according to claim 37 wherein a sub-step C4) offorming a barrier layer on top of said core region pattern, andoptionally on top of the lower cladding layer not covered by the coreregion pattern; is inserted before step D)
 40. A method according toclaim 38 wherein a sub-step C0) of forming a barrier layer on top ofsaid lower cladding layer is inserted before step C1).
 41. A methodaccording to claim 39 wherein a sub-step of annealing is inserted aftersaid barrier forming step or steps C0) and/or C4)
 42. A method accordingto claim 37 wherein the substrate is a silicon or quartz substrate. 43.A method according to claim 37 wherein the formation of layers on thesubstrate is made by plasma enhanced chemical vapour deposition.
 44. Amethod according to claim 43 wherein a standard cluster tool CVD processchamber type PECVD-apparatus from Surface Technology Systems is used forthe formation of layers on the substrate.
 45. A method according toclaim 43 wherein processing parameters of the PECVD-process areoptimized with a view to minimizing the optical absorption around λ=1508nm.
 46. A method according to claim 43 wherein processing parameters tobe optimized include one or more of the following: a) SiH₄ flow; b) theN₂O flow; c) the N₂ flow; d) the NH₃ flow; e) the power; f) thepressure; g) the temperature; h) the frequency; i) the flow or flowscomprising the element or elements X;
 47. A method according to claim 43wherein a) the SiH₄ flow rate is in the range from 0 to 30 sccm, such as10 to 30 sccm; b) the N₂O flow rate is in the range from 0 to 1000 sccm,such as 100 to 400 sccm; c) the N₂ flow rate is in the range from 0 to3000 sccm, such as 1000 to 3000 sccm. d) the NH₃ flow-rate is in therange from 0 to 300 sccm, such as 150 to 250 sccm; e) the power is inthe range from 0 to 1000 W, such as 400 to 1000 W. f) the pressure is inthe range from 100 to 500 mTorr, such as 200 to 500 mTorr. g) thetemperature is in the range from 200 to 500° C., such as 200 to 400° C.h) the frequency is around 380 kHz or around 13.8 MHz.
 48. A methodaccording to claim 46 wherein the X=P and in i) the PH₃ flow is providedby PH₃ diluted in N₂ or another carrier gas.
 49. A method according toclaim 48 wherein in i) the PH₃ flow is provided by 5% PH₃ in N₂ with aflow rate of 0 to 50 sccm such as 2 to 20 sccm.
 50. A method accordingto claim 46 wherein X comprises P and in i) the PH₃ flow is provided byPH₃ diluted in N₂ or another carrier gas and wherein the PH₃ flow valueis used as a stress optimization parameter for the core region.
 51. Amethod according to claim 43 wherein processing parameters of the PECVDprocess essentially have the following values: a) SiH₄ flow rate 20sccm; b) the N₂O flow rate 100-400 sccm; c) the N₂ flow rate 2000 sccm;d) the NH₃ flow rate is 100 sccm; e) the power is 700 W; f) the pressureis 250 mTorr; g) the temperature 350° C.; h) the frequency is 380 kHz;i) 5% PH₃ in N₂ flow rate 10 sccm;
 52. A method according to claim 46wherein in step i) the flow gas is selected among the group of gasesSiH₄, SiF₄, SiCl₄, SiF₄, Si₂H₆, SiH₂Cl₂, SiCl₂F₂, SiH₂F₂, N₂O, NO, N₂,NO₂, O₂, H₂O, H₂O₂, CO, CO₂, N₂O, NO, N₂, NO₂, NH₃, N₂, B₂H₆, AlH₃, PH₃,H₂S, SO, SO₂, GeH₄, AsH₃, or combinations thereof.
 53. An optical devicecomprising an optical waveguide as defined in claim
 1. 54. An opticaldevice according to claim 53 comprising a branching component, such as asplitter or an arrayed waveguide grating.
 55. An optical deviceaccording to claim 53 comprising an optical duplexer or triplexer.